The spectrum of isopropyl alcohol in the millimeter region was recorded using a hybrid radio-spectrometer. A total of 1278 rotational and vibration-rotational transitions of its gauche-conformer were identified. Rotational constants and quartic centrifugal constants were refined. Sextic centrifugal constants and all six internal rotation constants were determined. The tunnel splitting parameter was refined. The reduced Hamiltonian of internal rotation is used for the first time in full format and describes sufficiently well the vibration-rotational spectrum of the gauche-conformer of isopropyl alcohol, which is complicated by internal rotation.Key words: radio-spectrometer, Hamiltonian, internal rotation, spectrum in the millimeter region.Introduction. Isopropanol, (CH 3 ) 2 CHOH, consists of two methyls and one hydroxyl that undergo internal rotation around the C-C and C-O single bonds, respectively. The rotational degree of freedom around the C-O bond gives rise to trans-and gauche-conformers. The gauche-conformer does not have a plane of symmetry and exists as two rotamers, the structures of which are mirror reflections of each other. They interconvert via tunneling of the hydroxyl H atom through the cis-barrier (the plane of symmetry of the methyls). A change of orientation of the OH group shifts considerably the internal axes and alters the direction of the dipole moment components relative to the principal axes of the molecule. This causes the rotational spectrum to become very complicated. It becomes impossible to describe it within the framework of the accepted model of a nonrigid asymmetric top. Internal rotation of these groups produces all possible rotation-vibrational interactions that perturb the rotational spectrum of isopropanol. Torsional vibrations relative to the cis-barrier of the hydroxyl H atom lift the double degeneracy of the torsion-vibrational levels, which results in the formation of two energetically nonequivalent torsion-vibrational symmetric (+) and antisymmetric (-) states.
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